For certain applications, microelectronic components have to operate in vacuum to achieve the desired performances. This is true, in particular, for non-cooled detectors for infrared imaging based on bolometric microbridges, called “microbolometers” hereafter. In the field of so-called “thermal” infrared detectors, it is indeed known to use one-dimensional or two-dimensional arrays of elements sensitive to infrared radiation, capable of operating at ambient temperature.
A thermal infrared detector conventionally uses the variation of the electric resistivity of a thermometric material, or also known as “bolometric” according to its temperature. The unit sensitive elements of the detector, or “bolometers”, are usually in the form of membranes, each comprising a layer of thermometric material, and suspended above a substrate, generally made of silicon, by support arms having a high thermal resistance. Such membranes, collectively called “retina”, especially implement a function of absorption of the incident radiation, a function of conversion of the power of the absorbed radiation into thermal power, and a thermometric function of conversion of the generated thermal power into a variation of the resistivity of the thermometric material, such functions being implementable by one or a plurality of different elements. Further, the support arms of the membranes are also electrically conductive and connected to the thermometric layer thereof, and means for sequentially addressing and biasing the thermometric elements of the membranes and means for forming electric signals usable in video formats are usually formed in the substrate having the membranes suspended thereabove.
Such a detector is for example described in document: “Uncooled amorphous silicon technology enhancement for 25 μm pixel pitch achievement”; E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820E.
To make thermal losses by gas convection, which would limit the quality of the detection, negligible, the sensitive retina is usually integrated in a hermetically sealed housing, or housing, under a very low pressure. The housing is provided with a window transparent to radiations of interest, usually having a wavelength between 8 and 14 micrometers. As a variation, each bolometer is integrated in a hermetically sealed micro-housing provided with such a window. The pressure level in the housing is typically adjusted so that losses by convection are smaller than losses by thermal conduction via the support arms, thus ensuring a fine detection. To achieve this, the gas pressure in the housing is thus usually lower than or equal to 10−2 mbar, and preferably lower than 10−4 mbar.
However, obtaining such a pressure level requires specific techniques for the hermetic sealing of the housing. Further, it can be observed that certain materials degas after the sealing into the inner space, or cavity, delimited by the housing. The maintaining of the initial low pressure level in the housing during the detector lifetime, typically 20 years, should thus be ensured despite the degassing of the surfaces and elements internal to the housing.
Referring to the example of FIG. 1, the vacuum sealing of a bolometric detection device usually uses the following sub-assemblies: a base 10 comprising a bottom 12 and lateral walls 14, formed in one piece, bolometric detector 16, usually formed of the sensitive retina integrated on the sense substrate, and a cap transparent to infrared radiation 18, or “window”, playing at the same time a role of mechanical protection, of hermetic closing of base 10, and of transparency to infrared radiation. Finally, a getter 20 is also housed in the housing to maintain a sufficient vacuum level despite the degassing of the elements in communication with inner space 22 of housing 24 defined by base 10 and window 18.
Base 10 is usually formed of an assembly of mainly metallic or ceramic materials, and also forms the electric interface of the detector with the outside of the housing by means of connector elements 26. Component 16 is fixed to the bottom of the housing, for example, by gluing, and connected to connection areas 28 reserved for this purpose in housing 24 by a wiring 30 known per se in the state of the art.
Window 18 is assembled on base 10 directly or indirectly by means of an intermediate part, the assembly being formed by a fluxless soldering having operating conditions which limit the degassing of the sub-assemblies just described during the sealing of the window to the base. As known per se, the fluxless soldering requires the presence of non-oxidized metal layers on window 18 and base 10, at the level of surfaces intended for the hermetic junction of these parts. The metal layers are thus usually formed of one or a plurality of layers, at least the last one thereof being made of a noble metal such as gold, or more seldom platinum. Fluxless soldering, which enables to bond metal elements together by atomic diffusion by means of a mechanical action, or more currently by heating until the metal seal at least partially melts, is well known per se and will thus not be described in detail. As known per se, to obtain a good connection between metal elements, for example by welding or soldering, it is preferably for the elements not to be oxidized at their surface. To achieve this, either a deoxidizing material or flux is used, to remove the oxide layer, or the metal elements are non-oxidizable.
Getter 20 is usually made of materials having a strong affinity for the main gas molecules likely to be emitted (degassed) by all the internal surfaces of cavity 22 of the housing. The getter is in particular selected to adsorb H2, N2, O2, H2O, and volatile carbon compounds (called organic) such as, for example, CH4. Typical materials used for the getter are, as well known, alloys based on elements Zr, Ti, Co, Fe, or Ba. Getter 20 usually appears either in the form of sintered blocks fixed to the inside of cavity 22, or in the form of one or of a plurality of thin layers deposited on strips, plates, or mineral substrates by means of evaporation or of cathode sputtering techniques, integrated in cavity 22 on assembly of component 16 in housing 24.
As known per se, a getter used in this type of application requires being activated to be able to adsorb the previously-indicated gases, the activation comprising making the getter surface reactive by means of an adapted thermal cycle carried out in vacuum. A getter 20 in the form of thin films, having a thickness in the range from one to a few micrometers generally requires a thermal activation at a much lower temperature than a sintered getter. Thus, known thin-film getters can be activated by simple heating of the device just described at a temperature in the range from 350° C. to 400° C. once the housing has been sealed.
Sintered getters require a temperature in the order of 800° C. or more, so that a general heating of the tight housing at this temperature would cause irreparable damage. The activation of a sintered getter is thus performed by Joule effect by means of electric connections provided for this purpose, which enables to substantially only heat the getter. The activation of such a getter however induces an intense radiation capable of damaging the bolometers. Further, a thin-film getter is usually preferred since it requires no electric connection accessible from the outside of the housing. This character simplifies the architecture, and thus the manufacturing process, and thus decreases the cost of the housing. Indeed, the connections crossing the base of the housing may raise tightness problems and induce significant design constraints due to the high intensities to be applied during the activation to generate the necessary high temperatures.
However, a thin-film getter is a constraint for the general device manufacturing method, since the getter efficiency is all the better as the activation temperature is high, even though it remains within a lower range, as specified. Actually, this specific thermal activation step, generally formed on the entire device, always defines the maximum temperature point to which the electronic component is submitted during its manufacturing cycle. In other words, the complete design of the component, and generally all the portions of the device, are actually directly dependent on the characteristics of the thin-film getter used.